#!/usr/bin/perl
use warnings;
use strict;

my $text = "Hallo\t\r\nWelt   foo";

my %symbols = (
    ord " "  => '[SPACE]',
    ord "\n" => '\n',
    ord "\t" => '\t',
    ord "\r" => '\r',
);

while ( $text =~ m/\G  (\S*)  (\s)  (\S*)  /gx ) {
    print $1, 
          $symbols{ ord($2) }, 
          $3;
}
print "\n";